The invention relates to a circuit arrangement for the dimmable operation of a fluorescent lamp, in particular for use in motor vehicles as instrument lighting. The prior art discloses corresponding circuit arrangements in which the fluorescent lamp is operated at an operating frequency. The effect achieved by the switching-on and -off of the operating frequency with an apparatus and thus the lamp at a dimming frequency which lies above the visual frequency of the human eye, is that the human eye is given the impression that the fluorescent lamp has a different brightness, depending on the pulse width of the dimming frequency. In order to adjust the lamp current through the fluorescent lamp, it is necessary either to provide an additional regulator or to use a resonant circuit which is stabilized in a complicated manner. The object of the invention, therefore, is to specify a circuit arrangement for dimming a fluorescent lamp which is constructed in a simple manner.
This object is achieved by virtue of the fact that by means of the apparatus which switches the operating frequency on and off at the dimming frequency, the supply voltage can simultaneously be switched on and off at a switching frequency and the lamp current is thereby adjustable, the switching frequency being greater than the operating frequency.
As a result of the refinement of the circuit with a push-pull converter for generating the operating frequency, a simply constructed functional realization of oscillator and regulator is achieved.
A push-pull converter constructed in a particularly simple manner is realized by a resonant circuit comprising a capacitance element and an inductance element, said resonance circuit being connected to a first pole of the supply voltage. Furthermore, the resonance circuit can be alternately connected via two switches directly or via a third switch to the second pole of the supply voltage. In this case, the two switches are connected by a respective terminal to the terminals of the capacitance element and/or of the inductance element. In this circuit, the fluorescent lamp may either be arranged in parallel with the inductance element and/or capacitance element or be supplied with the operating frequency via a transformer, the primary winding of the transformer advantageously forming the inductance element of the resonant circuit.
The use of electronic switches such as e.g. transistors or field-effect transistors constitutes a cost-effective solution for the switches.
A circuit arrangement with few components is realized by a circuit arrangement as claimed in claim 5.
The circuit arrangement as claimed in claim 6 specifies a particularly effective arrangement for regulating the lamp current which is nevertheless constructed in a simple manner and with few components.
The positive feedback device in the form of a coil applied to the same coil former as the inductance element can be produced in a simple manner and at the same time as the inductance element.
By virtue of the fact that the lamp-current desired value is predetermined as a function of the temperature of the fluorescent lamp or of the surroundings, a minimum brightness is achieved even at low temperatures.
Circuit arrangements according to the invention which are constructed in a particularly simple manner are specified in claims 9 and 12. The circuit can be realized with a low outlay on components particularly when a microprocessor is used for the control device, which microprocessor may even already be present for other tasks, for example in a combination instrument of a motor vehicle, and the brightness control according to the invention is used for the instrument lighting. Of course, the circuit can also be realized with a separate microprocessor or by means of switching gates.
A circuit arrangement in which the operating frequency of the fluorescent lamp approximately corresponds to the resonant frequency of the resonant circuit results in a virtually sinusoidal operating frequency with few harmonics. This reduces interference that may issue from the circuit, and thus increases the electromagnetic compatibility of the circuit.
As a result of the two switches being simultaneously switched on and subsequently off b before or at the beginning of the pulse train intermission, the current contained in the resonant circuit can be short-circuited and persistence of the fluorescent lamp can thus be reliably prevented. Inserting a series inductor between one pole of the supply voltage and the resonant circuit makes it possible for the current through the circuit to be additionally stabilized and kept sinusoidal.